South Sudan’s warring leaders have failed to reach a deal on ending their 15-month-old civil war.

South Sudan's warring leaders have failed to reach a deal on ending their 15-month-old civil war, Ethiopia's prime minister said Friday.

A statement said Sudan Sudan's President Salva Kiir and rebel leader Riek Machar missed a deadline to reach a peace agreement by midnight Thursday, and that further talks on Friday "did not produce the necessary breakthrough."

"This is unacceptable, both morally and politically," Hailemariam Desalegn said in the statement issued by the east African regional bloc IGAD, which has been trying to mediate a peace deal.

In the statement addressed the people of South Sudan, Hailemariam gave IGAD's harshest criticism yet of Kiir and Machar.

"Continuing a war flagrantly disregards the interests of you, the people. It is an abdication of the most sacred duty leaders have to you, their people: to deliver peace, prosperity and stability," he said.

"I asked them to be courageous in offering compromises and alternatives, rather than only reiterating old positions. Both regional and world leaders joined this call. Unfortunately, as the missed deadline shows, our pleas have not been heeded," he said.

Acknowledging IGAD's failure to broker peace, he said the "peace process must be reinvigorated and reformed" -- promising that the bloc, along with other African nations and global powers, would "implement a common plan and table a reasonable and comprehensive solution to end the crisis in South Sudan."

"We will assist the parties to make the compromises that have so far eluded them. We will use all influence at our disposal to convince those that remain intransigent," he said, alluding to mounting calls for sanctions and an arms embargo.

Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when Kiir accused his sacked deputy Machar of attempting a coup.

Over two dozen armed forces - including government soldiers and allied militia backed by Ugandan soldiers on one side, and a range of rebel factions on the other - have been battling it since.

Tens of thousands of people have died in the conflict, 1.5 million have been displaced and 2.5 million are in dire need of food aid in South Sudan, which declared independence from Sudan in 2011.